As electronic scanning becomes more useful for acquiring image information from documents in terms of speed of data acquisition and number of data points collected, a concurrent development of mechanical elements to take advantage of the electronic improvements has become necessary. Electronic scanning devices of the type contemplated usually provide a linear array of photosensitive elements arranged in a row, parallel to a document to be scanned and trasverse to the direction of relative motion of the document and the array. Because the photosensitive elements represent small areas of the document and relatively small movements will vary the image detected by the photosensitive elements, the image quality obtained in electronic scanning of documents is highly motion sensitive. Accordingly, movements of the scanning carriage must be free from vibrations caused by scanning movement, speed variations, or slippage during acceleration. However, the higher sample resolutions in data acquisition, or correspondingly smaller sample size desirable with the use of the electronic scanning devices, results in a corresponding increase in motion quality problems.
In the past, a large variety of scanning arrangements have been provided for the optical scanning systems in reproduction machines such as xerographic devices, where a portion of the optics systems, such as, for example, mirrors for reflecting light from a document to a photoreceptor, are supported on parallel rails and driven by a lead screw or pulley drive system. The support arrangement generally used provides low friction bearing members, often with a shape conforming to the rails, supporting the weight of the scanning member on the rails, and biased against the rail to maintain contact with a spring bias force applied, for example by a spring-loaded roll or wheel arranged on the opposing side of the rail from the bearing member. An alternative arrangement might provide a supporting carriage slidably supported on parallel rods, such as that shown by U.S. Pat. No. 4,346,984 to Kingsley, incorporated herein by reference, and driven by a lead screw drive arrangement connected to the carriage member. Heretofore, as shown in the Kingsley patent, scanning arrays have been commonly supported on a fixed surface, with the scanning optical arrangements used for directing light reflected from a document to the array. However, in devices such as that taught in U.S. patent application Ser. No. 943,280 filed Dec. 18, 1986, incorporated herein by reference, a scanning array is supported on a carriage for scanning movement across a platen.
Tri-roll drive arrangements are known in transmission systems as part of vibration-free drive apparatus for providing a driving force for movement of an element. In such arrangements, a driven roll is arranged in frictional driving contact with two adjacent, but non-abutting roll members biased tgether with a spring force, with either the driven roll or the idler rolls in frictional driving contact with the element to which motion is to be imparted. The resulting arrangement is substantially vibration free, without any slippage on start-up. Thus, as shown in U.S. Pat. Nos. 4,444,069 to Dangschat, 2,748,615 to Nistri, 2,577,190 to Hare, and British Pat. No. 150,447 to Estrade, elements may be driven in a uniform and vibration free manner by providing a transmission system wherein motion of a rotating shaft or pulley is transmitted to a driven member through a tri-roll drive arrangement.